The project is designed to examine biochemical changes which may take place during storage of bull sperm at -196 degrees C over an extended period of time. Changes will be correlated with fertility tests. Parallel studies will be conducted with rabbits where fertilization rates, implantation rates and birth rates will be examined. Semen will be treated in vitro in an attempt to remove certain sperm types (as dead ones) and enrich portions of the ejaculate. The effect on fertility will be tested. Finally selective filtration of spermatozoa in the female will be studied. Attention will be focused especially on cervical mucus and the tubo-uterine junction as filter sites, and the possible role of phagocytosis on selective removal of spermatozoa.